Accept French Postal Services identifiers in forms

[Chatewgne]

The form validation in FRSIRENENumberMixin is missing a special case from the SIRENE register.

The identifiers for Postal Services need a special validation process and are not currently validated.

Source (in french) : https://www.sirene.fr/static-resources/doc/lettres/lettre-16-novembre-2013.pdf